1. Field of the Invention
The present invention generally relates to transmitters for communication systems and, more particularly, to an Automatic Level Control (ALC) circuit for transmitter amplifiers in a burst communications system, such as Time-Division-Duplex (TDD) or Time-Division-Multiple-Access (TDMA) to control the power level to a specified tolerance regardless of ambient temperature variation.
2. Description of the Prior Art
With the rapid increase in users of mobile communications, the Federal Communications Commission (FCC) assigns frequency channels and controls broadcast power levels to help alleviate overcrowded conditions. The control of output power from cellular and personal communications systems must be very carefully monitored to comply with FCC regulations. A major problem encountered in complying with the output power regulations is controlling voltage fluctuations induced by temperature variations in the components of the power amplifier.
Conventional techniques for temperature stable ALC involves the use of a dual diode power scheme wherein the voltage variation due to the temperature of the detector diode is compensated for by a similar variation in a second diode placed in thermal proximity of the detector diode. As the ambient temperature changes, a corresponding voltage drop across the principle diode detector will result in a change in the detected voltage, an equal and opposite change will develop in the secondary diode's voltage drop to correct the temperature induced change in the principle detectors voltage. The net voltage from the dual diode detector is thus largely independent of the temperature effect. The dual diode temperature compensation technique requires that the diode pair be precisely matched to obtain good results. Often the diode pair has to be hand selected for proper matching making this technique expensive to implement. Even then,dissimilar characteristics still exist between the two closely matched diodes to render the dual diode technique inferior.